Condensation & damp explained
Why your windows run with water every morning
Short answer: the glass is the coldest surface in the room, so it is the first to give the water back — and it is the only surface in the house that shows you. Everywhere else that is doing the same thing is doing it silently. That is what this page is about, and it is not a fault you caused.
Warm air holds moisture. Cold air cannot hold as much, so when warm damp air touches a cold surface it has to give some of it back — and it gives it back as water, onto whatever that cold surface happens to be. Overnight your house cools down, and the coldest surface in almost any room is the glass: a few millimetres thick, with a January night on the other side of it.
So the water on your window came out of the air inside your own bedroom. It did not come through the window, it is not a leak, and it is not rising up from anywhere. That distinction matters because it decides what the fix is — and because it is the most common thing to get wrong. People spend serious money on a damp-proof course for a problem that is condensing out of the air a metre above it.
The glass is the messenger, not the problem
This is the part worth knowing, and it is why a wet window is worth paying attention to rather than just wiping.
Glass shows you the water for two reasons. It is the coldest surface in the room, so it is the first to fall below the temperature at which the air has to let go — and it does not absorb anything, so the water has nowhere to go except sit there in front of you.
Every other cold surface in that room is collecting the same water and saying nothing about it. Plaster absorbs. The external wall behind your wardrobe, the corner where two outside walls meet, the reveal beside the window, the wall behind the headboard, the top corner of an upstairs ceiling — every one of those runs colder than the middle of the room, every one is below dew point on the same night your window is, and every one takes it quietly for weeks. Then it goes black, and it looks like it happened overnight.
Which means the useful question is not how do I stop my window doing that. It is where else in this house is doing exactly the same thing, where I cannot see it yet. That is what a survey is for, and it is why the answer to a wet window is almost never a window.
Is mine normal, or is mine a problem?
A completely dry window in a hard frost is unusual, and some of this is simply what a house does in winter. Here is the honest line between the two.
Ordinary, and not worth spending money on
A light misting along the bottom of the glass on the coldest two or three nights of the year, gone by mid-morning. The bathroom window and the mirror after a shower, clearing within half an hour once the fan has run. Condensation on a brand new build or a fresh extension through its first winter, while the water used to build it dries out of the structure — that one genuinely is temporary and it genuinely does stop.
Telling you something, and worth acting on
Water every morning from October through to March. Enough of it to run down and stand on the sill, rather than just to notice. Black speckling on the frame, on the rubber seal, or on the plaster reveal beside it. Bedrooms worse than the rest of the house. A musty smell in a room that is not the bathroom. Anywhere you have pulled furniture away from an outside wall and found the wall behind it cold, marked, or damp to the touch.
One thing that is not us, and is not ventilation. If the water is between the two panes of a double-glazed unit — misted up on the inside, where you cannot wipe it — then the seal on that unit has failed and it is breathing outside air. No amount of ventilation will touch it, and nobody should sell you any. That is a glazing job, replacing the sealed unit, and it is usually not an expensive one.
Where all that water comes from
Almost everybody assumes there must be a leak somewhere, because the volumes feel impossible. They are not impossible. They are just you.
A household of four puts ten to fifteen litres of water into their home every day, purely by living in it. That is the standard figure the industry designs against, not something we have estimated, and it breaks down into things nobody would think twice about doing:
- Breathing. Two adults asleep for eight hours put out roughly two thirds of a litre, into a room with the door shut and the window closed. This is the one nobody ever thinks of, and it is most of why bedrooms are so often the worst room in the house.
- A shower. About 1.7 litres, nearly all of it within ten minutes, all of it into one small room.
- Drying washing indoors. Around two litres a load, whether it goes on an airer or through a tumble dryer that is not vented outside. In winter, with nowhere outdoors to hang it, this is very often the largest single source in the house.
- Cooking. An uncovered pan on a rolling boil puts out about a litre an hour — and a recirculating cooker hood removes none of it. It filters the grease and the smell, and returns the steam straight back into the kitchen.
None of that is a fault, and none of it is something anybody should stop doing. It is what a home is for. The only real question is whether the building can get rid of it again — and across most of the housing round here, it no longer can. The chimneys were capped, the draughty windows were replaced with sealed ones, and in a great many cases the replacements went in with no trickle vents. Every one of those changes was an improvement in its own right. Together they removed the ventilation the house was designed around, and nothing was put back in its place.
What actually fixes it, and what only feels like it does
Roughly in the order people try them, against how well each one works.
Wiping it up
Removes last night’s water and changes nothing at all about tonight’s. Worth doing, so it does not stand on the frame and rot it — but it is housekeeping rather than a fix. If you are doing it every morning, you have already established that you have a problem.
Opening the windows
Genuinely works, because it is real ventilation — that is rather the point. It also throws out the heat you have just paid to put into the room, which is why nobody keeps it up through a cold January, and why the mould reliably returns in the rooms where people stopped.
A dehumidifier
Works while it is running, in the room it is standing in, and uses electricity to do it. It earns its place for a specific job, like drying a room out after a leak. What it is really good at is proving the point: if you are emptying two litres out of the back of it every day, that is two litres a day that were in the air you were breathing.
Turning the heating up
Warmer surfaces sit further above dew point, so it does help — and a cold, unheated spare room is very often the worst room in a house for exactly that reason. But heating a whole house harder in order to manage moisture is the most expensive item on this list, and it does nothing whatsoever about the moisture itself.
Anti-mould paint and mould sprays
These treat the growth, not the water feeding it. Painting over mould without changing the conditions buys a few months, and then it comes back through — usually in exactly the same shape. If that has already happened to you once, that is a useful diagnosis in itself.
Ventilation that runs continuously
This is the actual answer, and the reasoning is unglamorous: the moisture is produced continuously, so it has to be removed continuously. Done properly it runs at a few watts, extracts at a rate somebody has measured rather than a rate printed on a box, and with heat recovery it hands back the large majority of the warmth in the air it throws out.
And sometimes the answer is a small one. Plenty of the homes we look at do not need a system. They need one decent fan that actually moves what it claims to, ducted properly, and left running on trickle instead of switched on for four minutes a day. We would far rather tell you that than sell you something bigger — the recommendation is the thing you judge us on, and we would like to be asked back.
Start with what you can actually see
Most people arrive here with a symptom rather than a diagnosis, and the symptom is usually enough to narrow it down. Each of these explains what is happening and what genuinely fixes it — no form, no email address, just the answer.
My windows are wet every morning
Water running down the glass from October to March, and pooling on the sill. What it means, when it is ordinary and when it is not — and why the glass is the only surface in the room honest enough to show you.
Read the explanationThe mould keeps coming back
You cleaned it, you sprayed it, you painted over it, and six weeks later it returned in the same shape. Why that shape is the diagnosis, why anti-mould paint runs out, and how to tell which of the three damps you have.
Read the explanationThere is mould in the bathroom, and a fan
A fan running is not a fan working. The eight things that go wrong — starting with the one you can check yourself today with a torch and a loft hatch, which is the worst of them and the most common.
Read the explanationIt started after we had work done
New windows, insulation, a loft conversion, a new kitchen. Not a coincidence and usually not a botched job — every trade did their own work correctly, and nobody was responsible for the air.
Read the explanationQuestions people ask about condensation
Because it depends on three things and you only share one of them. It takes moisture in the air, a surface cold enough for that moisture to condense on, and too little ventilation to remove it before it does. Two houses in the same terrace can differ on all three: how many people live there, whether washing is dried indoors, whether the chimneys are still open, whether the windows went in with trickle vents, how warm the house is kept and for how long, and whether anyone has insulated. A neighbour with an open flue and a household of two is running a different building from a household of five with every opening sealed. It is not a sign you are doing anything wrong, and it is not a sign your house is worse — it means the balance in your home has tipped and theirs has not.
The water on the glass is not, in itself. What it indicates can be. Persistent condensation means the air in that room is regularly reaching the point where moisture drops out onto cold surfaces, and the surfaces you cannot see — behind furniture, in the corners of external walls, in the reveals — are getting the same treatment without showing it. That is what mould grows on. The WHO guidance on dampness and mould links occupancy of damp or mouldy buildings to increased respiratory symptoms, respiratory infections and worsening of asthma, and UKHSA says the same and adds that the effect on mental health should not be underestimated. So: the wet window is a warning rather than a hazard, and it is a useful one because it arrives long before the mould does.
It will reduce it in the room the dehumidifier is standing in, for as long as it is running, and it will cost you electricity to do it. It does not deal with the rest of the house, it does not run while you are out, and it treats the symptom rather than the cause. Where a dehumidifier genuinely earns its place is drying a room out after a leak or a flood. It is also an excellent diagnostic tool: if you are emptying a litre or two out of the back of it every day, that is a litre or two a day that was sitting in the air you were breathing, and it tells you the scale of what needs removing continuously rather than in bursts.
Because two people spend eight hours in there with the door shut and the window closed, breathing. Two adults asleep for eight hours put out roughly two thirds of a litre of water. Bedrooms are also usually the coolest occupied rooms in a house, they are often on the top floor with a cold ceiling above them, and they frequently have a wardrobe pushed hard against an external wall — which stops the room air warming that patch of plaster and turns it into the coldest surface in the room after the glass. If you want one thing to try tonight for nothing: pull the wardrobe two inches off the wall so air can get behind it.
It works, and if you can do it consistently it will make a real difference. The problem is that it is not free and it is not automatic: you are throwing out air you have paid to heat, and almost nobody keeps it up through a genuinely cold spell. The rooms where people stop are the rooms where the mould comes back. Ventilation that runs on its own removes the moisture continuously at a few watts, does it whether or not anyone remembers, and in a heat recovery system it takes the warmth back out of the outgoing air before it leaves. That is the difference between a habit and a fix.
They work, and they are not optional in the way most people assume. An extract fan can only pull air out of a room if replacement air can get in somewhere — seal a house completely and a bigger fan simply pulls harder against nothing and moves less. Trickle vents are the background ventilation the rest of the system is designed around. They do let a small amount of cold air in, deliberately and in a controlled way, which is a fraction of what an open window costs you. If yours are taped over or painted shut, that is worth undoing before spending money on anything else.
Condensation appears on cold surfaces, is worst in winter, is worst in the rooms where moisture is made or where people sleep, and typically shows up high — ceiling corners, window reveals, the top of external walls, behind furniture. Penetrating damp comes through from outside and tends to be a defined patch that tracks a defect: a failed gutter, cracked render, a bridged cavity, a slipped tile. Rising damp comes up from the ground, stops at a fairly level line about a metre up, and is far rarer than the firms selling injected damp courses would have you believe. They look similar on a wall and have completely different remedies, so the only reliable way to tell is to measure — surface temperatures, air readings, and a look at what the existing ventilation is actually moving. Fitting ventilation to a leak fixes nothing, and injecting a damp course into a condensation problem fixes nothing either.
Not sure where to start?
Send us a message with what you have noticed at home. We will tell you honestly whether you need a survey, a system, or neither.
Or see what it costs — unlike most in this trade, we publish our prices.
Trained on the systems we install
Certificated — and here they are
Most trade sites show you a row of logos. These are the actual certificates, held by this business. Tap either one to read it, or download the PDF.
- 12-month workmanship guarantee On top of the manufacturer’s warranty, which we register in your name. If anything we installed fails because of the way we installed it, we come back and put it right. What it covers.
- Company no. 12092552 Registered in England & Wales. Trading as Cavalli Ventilation since July 2019, and in ventilation since 2003.
- Airflow measured on every install A calibrated anemometer at the grille, recorded and handed over. The only figure that counts is the one measured in your room.